1. Field of the Invention
The present invention relates to a filter medium, more particularly, to a filter medium for liquid filtration to remove waste solid contained in processing liquor of an electric discharge metal engraving or cutting process or to remove waste particulate efficiently and to reclaim pure water from process water used for cutting, polishing and etching of a wafer for production of IC's where ultra-pure water is required, or more generally to a filter medium for internal combustion engine oil, fuel, or the like.
2. Description of the Related Art
Heretofore, there have been used a fiber integrity formed of mixture of natural pulp and synthetic fibers and impregnated with a phenolic resin or the like, polyester nonwoven fabrics (spunbond) and the like, as liquid filter medium for filtering liquor such as effluent of an electric discharge metal processing, process water of IC production, and oil or fuel for internal combustion engines. Any of those filter mediums is not satisfactory as its filtering efficiency is low and life short.
Conventional filter mediums generally function to trap particles inside the filter medium, and make use of relatively coarse filter materials in order to lengthen life, so that the aimed filtration efficiency can not be attained initially until particles have filled the filter pores to some extent. When the pores come to be filled with particles, pressure loss increases and the liquid f low resistance goes up, resulting in either short life or degradation of filtration efficiency as particles once trapped are released.
A high performance filter making use a porous sheet made of fluorocarbon resins or the like is known, but it is so expensive that its use is limited to certain particular fields only. Such sheet is not suitable for a filter that has to treat a large amount of liquid like in the case of process water of electric discharge metal engraving and of IC production.
In order to solve these problems, there have been developed a filter medium utilizing fibrillated synthetic fibers (Japanese Patent Application Laid-open No.92011/1984). However, such fibers are fine and when they alone are formed into a sheet according to an ordinary wet-laid method, it is inevitable that a great amount of them f low away through a wirecloth or they cause clogging of it. Even if a medium is dare to be prepared, it is a very dense due to use of the fine fibers alone and therefore, the filtration resistance is so high that the medium can not be practically used even though the filtering efficiency is high.
When monofilament fibers of a relatively large diameter are incorporated in a filter medium so as to lower the filtration resistance, the flow-away loss of the fine fibrillated fibers at the sheet forming step increases more than that as mentioned above, so that the sheet obtained tends to be coarse, filtering efficiency goes down to a great extent, and a sufficient performance can not be attained.
A two-layered filter medium having a density gradient along Z-direction is disclosed in Japanese Patent Application Laid-Open Nos.268321/1986, 268325/1986, and 275495/1986. This filter medium is composed of two filtering layers in combination differing in density so as to attain improved balance between filtration efficiency and filter life. This filter medium is fundamentally different from the filter medium of the present invention in which filtering function is performed by the surface of the medium (substantially by the filtering layer).
When the pores are filled with particles, the pressure loss increases, the liquid flow is disturbed and the filter life shortened, or the particles captured inside of the filter flow away resulting in low filtration efficiency.